FIG. 37 shows a self-propelled mobile carriage 90 which is adapted to move over differences in level such as steps and which is provided with means 92 for adjusting the position of the center of gravity of the carriage by moving a weight portion 91. The center-of-gravity shifting means 92 enables the carriage 90 to move over a difference in level greater than one-half the diameter of its wheels, further preventing the carriage from falling down while moving over the level difference.
For the carriage to move over a greater difference in level, the center of gravity of the carriage must be shifted toward the opposite side to the wheels which are to be moved up to the higher level, namely toward the rear wheels if the front wheels are to be moved up to the higher level, or toward the front wheels if the rear wheels are to be moved up to the higher level.
Stated more specifically with reference to FIG. 37, when the rear wheels 94 are to be moved up onto a step 80 subsequently after the front wheels 93 are brought up onto the step 80, the center of gravity needs to be shifted toward the front wheels. However, even if the weight portion 91 is shifted forward, the carriage as moved up onto the step 80 is inclined rearward in its entirety, with the result that the position of the center of gravity of the entire carriage as projected immediately therebelow is located at a rear portion of a phantom line interconnecting the positions of the center of the front wheel and the center of the rear wheel as projected immediately therebelow as shown in FIG. 37. Thus, the rear wheels 94 are unable to move up onto the step 80 due to the gravity acting on the rear wheels 94.
In order to effectively shift the center of gravity, there is a need to increase the weight proportion of the weight portion 91 to the carriage 90, but this entails the problem of increasing in the weight of the carriage 90.
Further if the carriage advances onto a higher level in an oblique direction, one wheel is likely to rise or float in the air, failing to come into contact with the ground and bringing the carriage out of proper balance. Thus the carriage is unable to move over the difference in level or to run along with good stability.